In, Chapter 1. Introduction
In order to successfully enable/disable repositories I had to first run
$ yum clean all
before I proceeded with
$ sudo yum-config-manager --disable rhel-server-ost-6-preview-rpms
I guess something needed to update that the openstack product had been consumed by subscription manager.
If yum-config-manager didn't output details of the yum repository settings you know it did nothing.